The present invention generally relates to laser systems and more particularly, is concerned with a laser system that generates a beam that is safe to the human eye.
The use of lasers in recent years has been continually increasing. The greater the power of lasers, the more risk there is to the people who may come into contact with the system.
Specifically, when a collimated beam of visible light enters the eye cornea, it passes through or is otherwise absorbed by the vitreous humor. The portion of the beam that is not absorbed is focused by the eye lens onto the retina. Under normal conditions, the light energy is converted by the retina into chemical energy, stimulating optical sensations. Eye injury results because the focused high energy laser beam cannot be absorbed and causes damage to the retina. This damage does not occur when conventional sources of illumination are exposed to the eye because the light is emitted in all directions and produce a sizeable (rather than focused) image on the retina that can be safely absorbed. It has been determined in the industry that laser beams having a wavelength in the range of 1.5 um to 2.2 um is completely absorbed by the vitreous humor thereby alleviating any damage to the retina.
Laser system used as optical radar and communication transmitters in populated locations need to be operated so as to avoid eye damage.
Up to the present, eye-safe lasers generally had low efficiency. Two of the predominant eye-safe lasers are based on laser emission in erbium-doped solid-state host materials pumped by pulsed gas discharge lamps or laser diodes, or on frequency conversion of a neodymium laser using stimulated Raman scattering in a molecular gas such as methane. These devices have many shortcomings. The erbium lasers have low efficiency (typically less than 0.1%) owing to the low stimulated emission coefficient of the laser transition in erb um 3+ions at a 1.54 um output and to the low efficiency for optical pumping with the visible flash lamp. Further, the erbium laser can only be operated in a pulsed mode. Stimulated Raman conversion requires a cell containing a high pressure flammable gas. This gas is excited by a neodymium pump laser to emit stimulated radiation in the eye-safe region. The Raman conversion therefore is not amenable to continuous wave operation. In addition, since the Raman process deposits energy in the conversion medium, causing thermal distortions, the eye-safe Raman laser cannot be conveniently operated at high average power or repetition rate.
Consequently, a need exists for the availability of a laser system operating in an eye-safe frequency with acceptable efficiency, a high pulse repetition rate and high average output power.